Oh man, the first time I got back on a bike after a hiatus of about 20 years, I rode 3 miles and I was totally kaput. How on earth I managed to get home, I still don't quite know - things got a bit hazy...
Plenty of good advice here already, but I'll add my two pennyworth as well:
We all had to start somewhere. No need to be embarrassed.
Little and often is the key at the beginning. And don't diss utility rides, no matter how short. You have a hybrid, so buy a rack, some panniers plus a decent lock and get into the habit of using the bike for trips to the shops. Sometimes it's quicker to take the bike, especially if you have to waste time looking for a parking space. The beauty of doing stuff by bike is there's invariably something you can lock your bike to near where you have to be.
Don't push yourself too far too fast and allow time for recovery between rides. Plus things like walking helps with general fitness as well.
A good set of maps is always a worthwhile investment e.g. OS Landranger (1:50,000) and OS Explorer (1:25,000). Helps you plan rides, especially once you start going a bit further from home. A good, fun thing to do is hop on a train with the bike, ride a stop or two (or more!) and then cycle home.
Do make sure your saddle is in the right position (height, fore / aft, tilt). Get that wrong, and you'll soon know about it - there are plenty of "how to" guides to this floating around online. If your bum still hurts, then there are options. But right now, any ouch moments are most likely to be your body simply getting used to cycling.
At this time of year, do make sure you're dressed warmly enough. There is nothing more miserable than cycling when you are chilled through. It's far easier to be able to take a layer off than to wish you'd brought an extra one along in the first place.
Everyone falls off at some point. Just try not to do it in front of an audience with a backpack full of tomatoes...
But most of all, enjoy your new bike, and happy cycling.